The present invention relates to a developing unit of the magnetic brush type for an electrophotographic copy-machine, having a cylindrical magnetic applicator carrying on its outer surface particles of one--component-magnetic toner.
The applicator is rotated adjacent to the surface of the photoconductive element of the copy machine, which carries thereon an electrostatic latent image of the original to be copied.
The electrostatic force of attraction of the latent image causes the selective deposition of toner particles, coming from the applicator, onto the surface of the photoconductive element, against the action of the magnetic force of attraction between the applicator and the particles, so developing the latent image.
A draw back which is present by using one component magnetic-toner as developing powder is due to the tendency of the toner particles to stably agglomerate so forming clots having dimensions well greater than that of the single toner particle.
This tendency is affected by the ambient humidity and by the triboelectrical and magnetic characteristics of the toner particle.
Whichever are the causes of the clots formation, a clot has a magnetic force of attraction to the applicator which can be either much less or much greater than that of a single toner particle; in the first case it is possible that some zones of the latent image have an electrostatic charge insufficient to attract a clot and thus these zone are not developed. On the contrary in the second case it is possible that some zones of the photoconductive element be unduly developed thus causing in the copy undesired "background" coloration.